1. Technical Field
The present invention relates to a semiconductor integrated circuit, and in particular, to a clock modulation circuit for correcting a duty ratio and a spread spectrum clock generator including the same.
2. Related Art
In recent years, as an integration speed and components per chip of a semiconductor apparatus increase, an EMI (electromagnetic interference) problem occurs due to electromagnetic wave radiation in a semiconductor apparatus. In particular, while an operation frequency increases, a wavelength is gradually shortened, and the length of a wiring line in a connection circuit or a substrate becomes just as short as the wavelength of a high-frequency signal. In such a minute semiconductor integrated circuit, an interconnection part, such as the wiring line, serves as an antenna, and thus the amount of electromagnetic wave radiation increases. Further, in an electronic apparatus that uses a semiconductor apparatus, which operates at a high frequency, the electromagnetic waves that are radiated due to the above-described reasons cause mutual interference between electronic apparatuses and signal interference of communication apparatuses.
A general electronic apparatus prevents electromagnetic wave radiation by measuring electromagnetic waves and changing the arrangement of the circuit or by shielding the electronic apparatus with a steel plate.
But, with a rapid increase of use in the portable apparatus, a small and light-weight electronic apparatus is demanded. For this reason, the method of changing the arrangement of the circuit and the method of shielding the electronic apparatus with the steel plate also have limitations.
Thus, in the related art, there is suggested a spread spectrum clock generator (SSCG) that performs a spread spectrum processing in order to modulate an operation clock frequency of a semiconductor apparatus. This is disclosed in U.S. Pat. No. 6,501,307.
In U.S. Pat. No. 6,501,307, a load capacitor is provided in a delay modulator that generates a modulated clock, thereby sufficiently securing a cycle of a spectrum modulation signal.
However, in such a known spread spectrum clock generator, although the cycle of the spectrum modulation signal can be secured through clock modulation, the duty ratio of the clock is changed, which results in duty ratio distortion. The duty ratio distortion may cause a signal error to affect the entire electronic apparatus, resulting in an adverse affect on the system operation.